The relationship between the chemical state of Pd species and the catalytic activity for methane combustion on Pd/CeO2

Abstract

Three Pd/CeO₂ catalysts are synthesized by reduction-deposition and an impregnation method (IMP) to clarify how the chemical state of Pd influences the catalytic performance for CH₄ combustion. The results show that Pd/CeO₂ (HHA), using hydrazine hydrate as a reductant, shows much higher catalytic activity and stability for CH₄ combustion than Pd/CeO₂ (FA), using formaldehyde as a reductant, and Pd/CeO₂ (IMP). The results of our comprehensive characterization reveal that there are two different chemical states of Pd species on Pd/CeO₂: Pd²⁺ in the form of PdO particles and Pd^δ+ (2 < δ ≤ 4) in the form of Pd_xCe_1−xO₂, and the latter is mainly induced by the Pd species entering the lattice of CeO₂, which acts as a transfer channel of active oxygen species from the CeO₂ lattice to active Pd species. The activity of Pd/CeO₂ directly correlates to the surface concentrations of PdO and adsorbed oxygen. An increase in the amount of Pd_xCe_1−xO₂ would thicken the transition layer and decrease the replenishment rate of oxygen vacancies through diffusion of lattice oxygen. Meanwhile, the existence of more Pd_xCe_1−xO₂ could promote the capture of produced CO₂ on the CeO₂ around Pd species and decrease the catalytic activity.